U.S. patent application number 10/946853 was filed with the patent office on 2005-12-08 for maintaining and searching sets of cells in a wireless communication system.
Invention is credited to Amerga, Messay, Emani, Chalapathi Rao.
Application Number | 20050272425 10/946853 |
Document ID | / |
Family ID | 34970554 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050272425 |
Kind Code |
A1 |
Amerga, Messay ; et
al. |
December 8, 2005 |
Maintaining and searching sets of cells in a wireless communication
system
Abstract
For cell measurement, a wireless device categorizes cells whose
identities are known to the device into multiple sets. The wireless
device may obtain these cells from the system via signaling and/or
detect these cells via searches. Different sets of cells may be
associated with different levels of importance (e.g., for handoff),
require different amounts of processing for measurements, and so
on. Each set is associated with a particular measurement rate.
Cells deemed to be more important (e.g., for handoff) are measured
more frequently. Cells deemed to be less important and/or require
more processing for measurements (e.g., cells with unknown timing)
are measured less frequently. The wireless device performs searches
and makes measurements for the cells in each set at the measurement
rate selected for that set.
Inventors: |
Amerga, Messay; (San Diego,
CA) ; Emani, Chalapathi Rao; (San Diego, CA) |
Correspondence
Address: |
Qualcomm, NC
5775 Morehouse Drive
San Diego
CA
92121
US
|
Family ID: |
34970554 |
Appl. No.: |
10/946853 |
Filed: |
September 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60572912 |
May 19, 2004 |
|
|
|
Current U.S.
Class: |
455/436 ;
375/E1.005; 455/67.11 |
Current CPC
Class: |
H04B 2201/70702
20130101; H04W 48/16 20130101; H04B 1/70735 20130101; H04B 1/7083
20130101; H04W 24/00 20130101; H04W 36/30 20130101; Y02D 30/70
20200801; H04W 52/0206 20130101; H04W 52/0245 20130101; H04W 88/08
20130101 |
Class at
Publication: |
455/436 ;
455/067.11 |
International
Class: |
H04Q 007/20 |
Claims
1. An apparatus in a wireless communication system, comprising: a
controller operative to categorize a plurality of cells into a
plurality of sets, each set being associated with a particular rate
for performing measurements for cells in the set, and wherein the
plurality of sets are associated with a plurality of different
measurement rates; and a demodulator operative to perform
measurements for the cells in each set at the measurement rate for
the set.
2. The apparatus of claim 1, wherein the cells in the system are
operated asynchronously, and wherein at least one set includes
cells with unknown timing.
3. The apparatus of claim 1, wherein the plurality of sets include
a first set with at least one serving cell, a second set of
neighbor cells with known timing, and a third set of neighbor cells
with unknown timing.
4. The apparatus of claim 3, wherein the controller is further
operative to initiate measurements for the at least one serving
cell at a first rate, initiate measurements for the neighbor cells
with known timing at a second rate that is less frequent than the
first rate, and initiate measurements for the neighbor cells with
unknown timing at a third rate that is less frequent than the
second rate.
5. The apparatus of claim 4, wherein the plurality of sets further
include a fourth set with candidate cells deemed to be good
candidates for handoff, and wherein the controller is further
operative to initiate measurements for the candidate cells at a
fourth rate that is more frequent than the second rate.
6. The apparatus of claim 5, wherein the candidate cells include
neighbor cells in the second set with high received signal
quality.
7. The apparatus of claim 5, wherein the candidate cells include
cells recently removed from the first set.
8. The apparatus of claim 5, wherein the plurality of sets further
include a fifth set with detected cells that are not identified as
neighbor cells by the system, and wherein the controller is further
operative to initiate measurements for the detected cells at a
fifth rate that is more frequent than the third rate.
9. The apparatus of claim 8, wherein the controller is further
operative to limit the fifth set to a predetermined number of
detected cells and to purge cells in the fifth set, as needed,
based on detection time and received signal quality for the cells
in the fifth set.
10. The apparatus of claim 1, wherein the measurement rate for each
set is selected based on likelihood of the cells in the set being
assigned for communication.
11. The apparatus of claim 10, wherein the measurement rate for
each set is further selected based on amount of processing required
to perform measurements for the cells in the set.
12. A method of performing measurements for cells in a wireless
communication system, comprising: categorizing a plurality of cells
into a plurality of sets, each set being associated with a
particular rate for performing measurements for cells in the set,
and wherein the plurality of sets are associated with a plurality
of different measurement rates; and performing measurements for the
cells in each set at the measurement rate for the set.
13. The method of claim 12, wherein the plurality of sets include a
first set with at least one serving cell, a second set of neighbor
cells with known timing, and a third set of neighbor cells with
unknown timing, and wherein measurements for the at least one
serving cell are made at a first rate, measurements for the
neighbor cells with known timing are made at a second rate that is
less frequent than the first rate, and measurements for the
neighbor cells with unknown timing are made at a third rate that is
less frequent than the second rate.
14. The method of claim 13, wherein the plurality of sets further
include a fourth set with candidate cells deemed to be good
candidates for handoff, and wherein measurements for the candidate
cells are made at a fourth rate that is more frequent than the
second rate.
15. The method of claim 14, wherein the plurality of sets further
include a fifth set with detected cells that are not identified as
neighbor cells by the system, and wherein measurements for the
detected cells are made at a fifth rate that is more frequent than
the third rate.
16. An apparatus in a wireless communication system, comprising:
means for categorizing a plurality of cells into a plurality of
sets, each set being associated with a particular rate for
performing measurements for cells in the set, and wherein the
plurality of sets are associated with a plurality of different
measurement rates; and means for performing measurements for the
cells in each set at the measurement rate for the set.
17. The apparatus of claim 16, wherein the plurality of sets
include a first set with at least one serving cell, a second set of
neighbor cells with known timing, and a third set of neighbor cells
with unknown timing, and wherein measurements for the at least one
serving cell are made at a first rate, measurements for the
neighbor cells with known timing are made at a second rate that is
less frequent than the first rate, and measurements for the
neighbor cells with unknown timing are made at a third rate that is
less frequent than the second rate.
18. The apparatus of claim 17, wherein the plurality of sets
further include a fourth set with candidate cells deemed to be good
candidates for handoff, and wherein measurements for the candidate
cells are made at a fourth rate that is more frequent than the
second rate.
19. The apparatus of claim 18, wherein the plurality of sets
further include a fifth set with detected cells that are not
identified as neighbor cells by the system, and wherein
measurements for the detected cells are made at a fifth rate that
is more frequent than the third rate.
20. A processor readable media for storing instructions operable in
a wireless device to: categorize a plurality of cells into a
plurality of sets, each set being associated with a particular rate
for performing measurements for cells in the set, and wherein the
plurality of sets are associated with a plurality of different
measurement rates; and initiate measurements for the cells in each
set at the measurement rate for the set.
21. An apparatus in a wireless communication system, comprising: a
demodulator operative to perform searches to detect for cells in
the system; and a controller operative to identify cells detected
by the searches and not signaled by the system, store the detected
cells in a set, and purge cells in the set, if needed, to limit the
set to a predetermined number of detected cells.
22. The apparatus of claim 21, wherein the controller is further
operative to determine whether to purge any cells currently stored
in the set in order to make room for cells detected in a current
measurement period, and if cells are to be purged from the set,
identify at least one cell to purge based on last detection time
and received signal quality for the cells currently stored in the
set.
23. The apparatus of claim 22, wherein the controller is further
operative to sort the cells currently stored in the set based on
last detection time and received signal quality for the cells,
select cells for purging starting with oldest detection time, and
if multiple cells have equal detection time, select cells for
purging starting with lowest received signal quality.
24. A method of maintaining cells in a wireless communication
system, comprising: performing searches to detect for cells in the
system; identifying cells detected by the searches and not signaled
by the system; storing the detected cells in a set; and purging
cells in the set, if needed, to limit the set to a predetermined
number of detected cells.
25. The method of claim 24, further comprising: determining whether
to purge any cells currently stored in the set in order to make
room for cells detected in a current measurement period; and if
cells are to be purged from the set, identifying at least one cell
to purge based on last detection time and received signal quality
for the cells currently stored in the set.
26. An apparatus in a wireless communication system, comprising:
means for performing searches to detect for cells in the system;
means for identifying cells detected by the searches and not
signaled by the system; means for storing the detected cells in a
set; and means for purging cells in the set, if needed, to limit
the set to a predetermined number of detected cells.
27. The apparatus of claim 26, further comprising: means for
determining whether to purge any cells currently stored in the set
in order to make room for cells detected in a current measurement
period; and means for, if cells are to be purged from the set,
identifying at least one cell to purge based on last detection time
and received signal quality for the cells currently stored in the
set.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S.
Provisional Application Patent No. 60/572,912 filed May 19,
2004.
BACKGROUND
[0002] I. Field
[0003] The present invention relates generally to communication,
and more specifically to techniques for making signal quality
measurements for cells in a wireless communication system.
[0004] II. Background
[0005] In a wireless communication system, a wireless device may
communicate with one or multiple "cells" at any given moment. A
cell can refer to a base station or the coverage area of the base
station, depending on the context in which the term is used. The
cells in the system may be categorized as "serving" and
"non-serving" cells. A serving cell is a cell that the wireless
device is in communication with or is designated to receive
messages from. A non-serving cell is a cell that is not a serving
cell. A "neighbor" cell is a non-serving cell that may be near a
serving cell and may be received by the wireless device.
[0006] The wireless device periodically makes signal quality
measurements for the serving cell(s) as well as other cells in the
system. The wireless device makes these measurements in order to
determine whether there are any cells better than the current
serving cell(s). This may be the case, for example, if the wireless
device is mobile and moves about the system. If a better cell is
found, as indicated by the measurements, then the wireless device
may be handed from the current serving cell(s) over to the better
cell, which would then become the new serving cell.
[0007] Cell measurements are important to ensure good performance
for the wireless device and achieve high system efficiency.
However, these measurements consume valuable resources (e.g.,
battery power) at the wireless device. There is therefore a need in
the art for techniques to make cell measurements in an efficient
manner in order to conserve resources.
SUMMARY
[0008] Techniques for making cell measurements in a wireless
communication system are described herein. A wireless device
categorizes cells whose identities are known to the device into
multiple sets. The wireless device may obtain these cells from the
system via signaling and/or detect these cells via searches.
Different sets of cells may be associated with different levels of
importance (e.g., for handoff), require different amounts of
processing for measurements, and so on. Each set is associated with
a particular measurement rate, which may be selected based on the
characteristics of the set. For example, cells deemed to be more
important (e.g., serving cells with which the wireless device
communicates, or cells that are good candidates for serving the
wireless device) are measured more frequently. Cells deemed to be
less important and/or require more processing for measurements
(e.g., cells with unknown timing) are measured less frequently. The
wireless device performs searches and makes measurements for the
cells in each set at the measurement rate selected for that
set.
[0009] Various aspects and embodiments of the invention are
described in further detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The features and nature of the present invention will become
more apparent from the detailed description set forth below when
taken in conjunction with the drawings in which like reference
characters identify correspondingly throughout and wherein:
[0011] FIG. 1 shows a wireless communication system;
[0012] FIG. 2 shows a state diagram for five exemplary sets of
cells;
[0013] FIG. 3 shows a timeline for performing searches for the five
cell sets;
[0014] FIG. 4 shows a process for performing searches for the cells
in the five sets;
[0015] FIG. 5 shows a process for maintaining a set of detected
cells;
[0016] FIG. 6A shows a frame structure used by Wideband-CDMA
(W-CDMA);
[0017] FIG. 6B shows a primary common pilot channel (CPICH) and a
synchronization channel (SCH) for the downlink in W-CDMA; and
[0018] FIG. 7 shows a block diagram of a wireless device.
DETAILED DESCRIPTION
[0019] The word "exemplary" is used herein to mean "serving as an
example, instance, or illustration." Any embodiment or design
described herein as "exemplary" is not necessarily to be construed
as preferred or advantageous over other embodiments or designs.
[0020] The cell measurement techniques described herein may be used
for various wireless communication systems such as Code Division
Multiple Access (CDMA) systems, Time Division Multiple Access
(TDMA) systems, and Frequency Division Multiple Access (FDMA)
systems. A CDMA system may implement W-CDMA, cdma2000, or some
other CDMA radio access technology (RAT). cdma2000 covers IS-2000,
IS-856, and IS-95 standards. A TDMA system may implement Global
System for Mobile Communications (GSM) or some other TDMA RAT.
W-CDMA and GSM are described in documents from a consortium named
"3.sup.rd Generation Partnership Project" (3GPP). cdma2000 is
described in documents from a consortium named "3.sup.rd Generation
Partnership Project 2" (3GPP2). 3GPP and 3GPP2 documents are
publicly available. The cell measurement techniques may also be
used to measure cells for one or multiple wireless systems. For
clarity, these techniques are described below for a W-CDMA
system.
[0021] FIG. 1 shows a wireless communication system 100. System 100
includes a number of base stations 110 that support communication
for a number of wireless devices 120. A base station is a fixed
station used for communicating with the wireless devices and may
also be called a Node B (W-CDMA terminology), a base transceiver
station (BTS), an access point, or some other terminology. Wireless
devices 120 are typically dispersed throughout the system, and each
wireless device may be fixed or mobile. A wireless device may also
be called a user equipment (UE) (W-CDMA terminology), a mobile
station, a user terminal, or some other terminology.
[0022] In FIG. 1, a solid line with arrows on both ends indicates
active communication between a wireless device and a base station.
A dashed line with an arrow on one end indicates reception of pilot
by a wireless device from a base station. A wireless device may
communicate with one base station (e.g., wireless device 120a) or
multiple base stations (e.g., wireless device 120g) on the downlink
and uplink at any given moment. The downlink (or forward link)
refers to the communication link from the base stations to the
wireless devices, and the uplink (or reverse link) refers to the
communication link from the wireless devices to the base
stations.
[0023] System 100 may include many cells, where a cell may refer to
a base station and/or its coverage area. A wireless device may be
in active communication with one or multiple serving cells in a
"connected" or "dedicated" mode of operation. The wireless device
may also be designated to receive messages (e.g., pages) from one
or multiple serving cells while in an "idle" mode. In any case, the
wireless device may be able to receive signals from other cells
besides the serving cell(s). For clarity, the following description
is for the connected mode. However, the description also generally
applies for the idle mode.
[0024] In W-CDMA as well as many other wireless systems, a wireless
device performs initial acquisition and searches for cells when the
device is first powered on. If any cells are found, then the
wireless device exchanges signaling with these cells to inform the
system of the device's presence and, if necessary, to set up a
call. Via this signaling exchange, the wireless device obtains (1)
an active set containing one or more cells to use for the call and
(2) pertinent parameters for the radio link for each cell in the
active set. The active set may contain one to six cells in W-CDMA.
The cells in the active set are typically selected by the system
based on inputs from the wireless device.
[0025] The wireless device may also obtain a list of neighbor
cells, which are cells that may be received by the wireless device.
This list may be called a neighbor list, a monitored set, and so
on. In W-CDMA, the serving cell forms the neighbor list and sends
the list to the wireless device via signaling. The neighbor list
may contain up to 32 "intra-frequency" cells operating on the same
frequency channel as the serving cell(s), up to 32
"inter-frequency" cells operating on frequency channels different
from the frequency channel of the serving cell(s), and up to 32
inter-RAT cells of different radio access technology (e.g., GSM).
Each cell also broadcasts a list of neighbor cells for that cell.
The wireless device may then form a neighbor list for itself based
on neighbor cell information obtained from all serving cells. In
any case, depending on how the system is operated, the neighbor
list obtained by the wireless device may be (1) fairly
comprehensive and include many or all of the pertinent neighbor
cells that may be received by the wireless device or (2) somewhat
incomplete and omit some pertinent neighbor cells. The wireless
device may thus detect neighbor cells that are not included in the
neighbor list.
[0026] The wireless device may perform a "full" search to detect
for the presence of a cell, determine the cell's timing, and make
measurement for the cell. This full search is dependent on the
system design, e.g., on how pilot and synchronization information,
if any, are transmitted by each cell in the system. An exemplary
full search for W-CDMA is described below. For most systems
including W-CDMA, the full search is computationally intensive. In
addition, for W-CDMA, the cells in the system may be operated
asynchronously so that each cell may transmit on the downlink based
on its timing, which may be different from the timing of other
cells. For an asynchronous system, the wireless device would need
to ascertain the timing of each individual cell.
[0027] The wireless device may perform a "list" search to make
measurement for a cell whose timing is known to the wireless
device. The list search is also dependent on the system design but
is typically less computationally intensive than the full search.
An exemplary list search for W-CDMA is described below. The list
search can ascertain changes in the cell timing and discover new
signal paths that may have been formed since the last measurement
for the cell.
[0028] The wireless device periodically makes measurements for the
serving cell(s) as well as non-serving cells in order to determine
the best cell(s) from which to receive service. The wireless device
may perform either a full search or a list search to make
measurement for a cell, depending on whether timing for the cell is
known. The terms "search" and "measurement" are thus related in the
context of cell measurement and may be used interchangeably. The
wireless device may be able to quickly and easily make measurement
for a cell whose timing is known. The wireless device may need to
perform a full search in order to make measurement for a cell whose
timing is not known. The wireless device may thus expend different
amounts of resources to make measurements for different cells.
[0029] Table 1 shows an embodiment of five different sets of cells
that may be maintained by the wireless device.
1TABLE 1 Cell Set Description Active set Contains serving cells
that have assigned dedicated resources to the wireless device.
Candidate set Contains cells from the Known Timing Neighbor set and
the Unlisted set and which are strongly received by the wireless
device. Known Timing Contains neighbor cells for which the wireless
device Neighbor set has timing information. Unknown Timing Contains
neighbor cells for which the wireless device Neighbor set does not
have timing information. Unlisted set Contains cells that are
detected by the wireless device but are not specified as neighbor
cells by the network. The cells in this set are also called
"detected" cells.
[0030] Each cell that is identified by the wireless device may be
categorized in one of the cell sets listed in Table 1. In general,
fewer, more, and/or different cell sets may be defined, and this is
within the scope of the invention. For example, multiple sets may
be formed for candidate cells with different received signal
quality.
[0031] FIG. 2 shows an exemplary state diagram 200 for the five
cell sets shown in Table 1. The wireless device obtains a set of
serving cells and a list of neighbor cells from the system for a
call. The serving cells are placed in the Active set 210, and the
neighbor cells in the neighbor list are initially placed in the
Unknown Timing Neighbor set 250. The wireless device may perform a
full search to detect for each neighbor cell in the Unknown Timing
Neighbor set. The wireless device moves each neighbor cell that is
detected by the full search to the Known Timing Neighbor set 230.
In the process of performing full searches for neighbor cells, the
wireless device may detect other cells that are not included in the
neighbor list but are nevertheless received with sufficient signal
quality by the wireless device. The wireless device places these
detected cells in the Unlisted set 240.
[0032] The wireless device periodically makes measurements for
cells that have been detected by the device and for which timing
information is known to the device. The wireless device may move
cells in the Known Timing Neighbor set and Unlisted set having
sufficiently high received signal quality to the Candidate set 220.
The wireless device may also send measurement reports to inform the
system of these strong cells. For W-CDMA, the transition from the
Known Timing Neighbor set and the Unlisted set to the Candidate set
may be triggered by events 1A, 1C, 1D, and 1E, which are described
below.
[0033] The system may update the cells in the Active set based on
measurement reports sent by the wireless device. New cells (e.g.,
in the Candidate set) may be added to the Active set by performing
an active set update (ASU) procedure for radio link addition.
Conversely, current serving cells in the Active set may be removed
by performing an active set update procedure for radio link
removal. The active set update procedures for W-CDMA are described
in a document 3GPP TS 25.331, entitled "RRC Protocol
Specification," which is publicly available.
[0034] The wireless device may also place cells that have recently
been removed from the Active set into the Candidate set. This is
because (1) the wireless device was recently in communication with
these cells and (2) there is higher likelihood of the wireless
device communicating with these cells in the future. The wireless
device may move cells in the Candidate set back to the Known Timing
Neighbor set or the Unlisted set if the received signal quality for
these cells is not sufficiently high. For W-CDMA, the transition
from the Candidate set to the Known Timing Neighbor set and the
Unlisted set may be triggered by events 1B and 1F, which are
described below.
[0035] The wireless device may report the detected cells in the
Unlisted set to the system by sending a Measurement Control Message
(MCM) to the serving cell(s). If the system adds these detected
cells to the neighbor list, then the wireless device moves these
cells to the Known Timing Neighbor set. Conversely, if the system
removes cells from the neighbor list but the wireless device
continues to receive these cells with high received signal quality,
then the wireless device may move these cells to the Unlisted set.
If the system does not select cells outside of the neighbor list as
serving cells, then the wireless device may keep the detected cells
in the Unlisted set instead of moving these cells to the Candidate
set.
[0036] In W-CDMA, each cell transmits a continuous pilot on a
primary CPICH. This pilot is channelized (or orthogonalized) using
a known channelization code assigned to the primary CPICH and
further spectrally spread (or scrambled) with a primary scrambling
code assigned to the cell. If the timing of the cell is known, then
the wireless device can perform the complementary spectral
despreading (or descrambling) and then make measurement for the
signal quality of the pilot received on the primary CPICH. The
received signal quality is also called received signal strength,
received pilot strength, and so on, and may be quantified by an
energy-per-chip-to-total-noise ratio (Ec/No) or some other
quantity.
[0037] The wireless device may compare the received signal quality
for the primary CPICH for a given cell against one or more values
to ascertain the quality of that cell. For example, the wireless
device may compare the received signal quality for the cell against
a reporting range constant sent by the cell, an absolute threshold,
the received signal quality for a serving cell, and so on.
[0038] Table 2 lists an exemplary set of events that may trigger
transition between the Candidate, Known Timing Neighbor, and
Unlisted sets. A cell in the Known Timing Neighbor set or the
Unlisted set may transition to the Candidate set if the received
signal quality for the primary CPICH for that cell is (1) higher
than the reporting range constant (event 1A), (2) better than the
primary CPICH for a serving cell (event 1C), (3) better than the
primary CPICH for a previously best cell (event 1D), or (4) higher
than the absolute threshold (event 1E). A cell in the Candidate set
may transition back to the Known Timing Neighbor set or the
Unlisted set if the primary CPICH for that cell is (1) lower than
the reporting range constant (event 1B) or (2) lower than the
absolute threshold (event 1F). The wireless device may send a
measurement report to the serving cell(s) whenever any one of
events 1A through 1F occurs. The wireless device may also
periodically send measurement reports for cells that triggered
events 1A and 1C. Events 1A through 1F are all based on received
signal quality and are described in the aforementioned 3GPP
document 3GPP TS 25.331.
2TABLE 2 Event Description 1A Primary CPICH enters the reporting
range. 1B Primary CPICH leaves the reporting range. Primary CPICH
for a non-serving cell becomes better than primary 1C CPICH for a
serving cell. 1D Change of best cell. 1E Primary CPICH becomes
better than the absolute threshold. 1F Primary CPICH becomes worse
than the absolute threshold.
[0039] State diagram 200 also shows an exemplary hierarchy of
various cells identified by the wireless device (e.g., based on
signaling from the system and/or measurement by the wireless
device). The serving cells in the Active set may be deemed the most
important since they are the ones with which the wireless device
communicates. The cells in the Candidate set may be deemed the next
most important since they are good candidates for serving the
wireless device. The candidate cells may be selected based on their
current received signal quality, their past status (e.g., as recent
serving cells), and/or other factors. The cells in the Known Timing
Neighbor set and the Unlisted set may be deemed less important
since they have not met the criterion or criteria for inclusion in
the Candidate set. However, these cells may grow in importance,
e.g., if the wireless device moves into the coverage of these
cells. The cells in the Known Timing Neighbor set and the Unlisted
set may be deemed to be equal in importance, as shown in FIG. 2.
Alternatively, the cells in the Known Timing Neighbor set may be
given greater importance than the cells in the Unlisted set, e.g.,
if the system only selects cells in the neighbor list for the
Active set. The cells in the Unknown Timing Neighbor set may be
deemed to be least important of all the cells identified by the
wireless device. This is because not much information may be
available for these cells except for their identities.
[0040] The wireless device may make measurements for different
cells at different rates. Cells deemed to be more important may be
measured more frequently, as indicated in FIG. 2. Cells that
require more processing for measurement (e.g., for a full search)
may be measured less frequently. The rate of measurement for each
set of cells may be determined based on various factors such as the
importance of the cells in the set, the amount of processing needed
to perform the measurement, and so on. More interesting or
pertinent cells may be measured more often.
[0041] The wireless device may perform a search for each serving
cell at a frequent rate of R.sub.1 to identify the best signal
paths for processing. The wireless device may perform a search for
each candidate cell at a frequent rate of R.sub.2 to ensure that
good candidates for (soft or hard) handoff are reported quickly to
the system. The wireless device may perform a search for each cell
in the Known Timing Neighbor set at a less frequent rate of R.sub.3
to look for worthy cells to add to the Candidate set. The wireless
device may perform a search for each cell in the Unlisted set at a
rate of R.sub.4 to look for cells to add to the Candidate set. The
wireless device may perform a search for each cell in the Unknown
Timing Neighbor set at an even less frequent rate of R.sub.5 to
look for cells identified by the system as well as cells not
identified by the system.
[0042] In general, any measurement rate may be used for each cell
set. The measurement rates for the five cell sets in FIG. 2 may
conform to the following guideline:
R.sub.1.gtoreq.R.sub.2.gtoreq.R.sub.3.gtoreq.R.sub.4-
.gtoreq.R.sub.5. As a specific example, the wireless device may
perform searches for serving cells at a rate of 50 searches per
second (sps), i.e., R.sub.1=50 sps, which is one search in every
measurement period or interval of P.sub.1=20 milli-seconds (ms) for
each serving cell. The wireless device may also perform searches
for candidate cells at a rate of R.sub.2=50 sps. The wireless
device may perform searches for cells in the Known Timing Neighbor
set and the Unlisted set at a rate of R.sub.3=R.sub.4=12.5 sps, or
one search in every measurement period of P.sub.3=P.sub.4=80 ms for
each cell in these sets. The wireless device may perform searches
for cells in the Unknown Timing Neighbor set at a rate of
R.sub.5=3.125 sps, or one search in every measurement period of
P.sub.5=320 ms for each cell in this set. Other measurement rates
may also be used for these cell sets.
[0043] FIG. 3 shows an exemplary timeline for performing searches
for cells in the five sets shown in FIG. 2. In this example, the
wireless device performs searches for the serving and candidate
cells at the same rate, so that R.sub.1=R.sub.2 and
P.sub.1=P.sub.2. The wireless device also performs searches for
cells in the Known Timing Neighbor set and the Unlisted set at the
same rate, so that R.sub.3=R.sub.4 and P.sub.3=P.sub.4. At time
T.sub.a, the wireless device performs (1) list searches for the
serving cells in the Active set, the candidate cells in the
Candidate set, the neighbor cells in the Known Timing Neighbor set,
and the detected cells in the Unlisted set and (2) full searches
for the neighbor cells in the Unknown Timing Neighbor set. The
wireless device performs searches for the serving and candidate
cells every P.sub.1 ms thereafter (e.g., at time T.sub.b). The
wireless device performs searches for the cells in the Known Timing
Neighbor set and the Unlisted set every P.sub.3 ms thereafter
(e.g., at time T.sub.c). The wireless device performs full searches
for the cells in the Unknown Timing Neighbor set every P.sub.5 ms
thereafter (e.g., at time T.sub.d).
[0044] FIG. 3 shows the wireless device performing searches for all
cells in each set at approximately the same time, or lumped
together at times T.sub.a, T.sub.b, T.sub.c and T.sub.d. The
searches for the cells in each set may also be distributed over the
measurement interval so that the processing is spread over time.
For example, if the active set contains N.sub.1 serving cells, then
the wireless device may perform a search for a different serving
cell every P.sub.1/N.sub.1 ms, so that all N.sub.1 serving cells
are searched every P.sub.1 ms. The wireless device may similarly
distribute the searches for the cells in each of the other
sets.
[0045] FIG. 4 shows a flow diagram of a process 400 for performing
searches for cells in the five sets shown in FIG. 2. The wireless
device receives the active set and the neighbor list from the
system and initializes the Active set and the Unknown Timing
Neighbor set with the cells in the active set and the neighbor
list, respectively (block 412). The Active set and Unknown Timing
Neighbor set may also be initialized in other manners. The wireless
device may initialize the other three cell sets to null or
empty.
[0046] The wireless device thereafter performs measurement for the
cells in the various sets at the rate selected for each set. If the
time for making measurements for the Active set has arrived (as
determined in block 414), then the wireless device makes
measurements for the serving cell(s) in the Active set (block 416)
and then proceeds to block 434. If the measurement time for the
Candidate set has arrived and this set is not empty (as determined
in block 418), then the wireless device makes measurements for the
candidate cells (block 420) and then proceeds to block 434. If the
measurement time for the Known Timing Neighbor set has arrived and
this set is not empty (as determined in block 422), then the
wireless device makes measurements for the neighbor cells in this
set (block 424) and then proceeds to block 434. If the measurement
time for the Unlisted set has arrived and this set is not empty (as
determined in block 426), then the wireless device makes
measurements for the detected cells in this set (block 428) and
then proceeds to block 434. If the measurement time for the Unknown
Timing Neighbor set has arrived and this set is not empty (as
determined in block 430), then the wireless device makes
measurements for the neighbor cells in this set (block 432) and
then proceeds to block 434. If the measurement time for none of the
cell sets has arrived (as determined in blocks 414, 418, 422, 426,
and 430), then the wireless device proceeds to block 438.
[0047] In block 434, the wireless device receives measurement
results and moves the cells among the sets, as appropriate, based
on the measurement results for these cells (e.g., as shown in FIG.
2). The wireless device also generates and sends measurement
reports, if appropriate (block 436). The wireless device then
determines whether new cell information (e.g., a new active set or
neighbor list) has been received from the system (block 438). If
the answer is `yes`, then the wireless device returns to block 412
and reinitializes the cell sets. Otherwise, the wireless device
returns to block 414 and continues to make cell measurement.
[0048] FIG. 4 shows a specific embodiment for performing cell
measurements with multiple sets. Block 412 initializes the cell
sets. Blocks 414 through 432 categorize the cells in the system and
perform measurements for cells in different sets at different rates
selected for these sets. The cell measurements may also be
performed in other manners different from that shown in FIG. 4.
Block 434 updates the cell sets based on the measurement
results.
[0049] The Unlisted set contains cells that are not included in the
neighbor list but are nevertheless detected by the wireless device.
For W-CDMA, there are 8192 possible scrambling codes, of which 512
are primary scrambling codes. The number of detected cells may
potentially be large. To reduce complexity, the Unlisted set may be
limited to L detected cells, where L may be any value, e.g.,
L=10.
[0050] FIG. 5 shows a flow diagram of a process 500 for maintaining
the Unlisted set. The wireless device initializes the Unlisted set
to null or empty (block 512). The wireless device then waits until
the next measurement period for the Unlisted set or the Unknown
Timing Neighbor set (block 514).
[0051] When the next measurement period arrives, the wireless
device performs searches and identifies at most L detected cells
for this measurement period (block 516). The wireless device then
determines the number of cells, if any, to be purged from the
Unlisted set (block 518). The number of cells to purge is
determined by (1) the number of newly detected cells for the
measurement period and (2) the number of cells already stored in
the Unlisted set at the start of the measurement period. The
wireless device may populate the Unlisted set with cells detected
by the searches until the number of detected cells in the set
reaches L. Once the Unlisted set reaches the limit of L, the
wireless device purges the oldest and/or weakest detected cells
from the set in order to make room for newly detected cells.
[0052] If purging is needed, as determined in block 520, then the
wireless device identifies the cell(s) to purge based on the
detection time and/or received signal quality for the L cells
currently stored in the Unlisted set (block 522). The wireless
device then purges the identified cell(s) from the Unlisted set
(block 524). After purging the identified cell(s) or if purging is
not needed, the wireless device stores the detected cells for the
measurement period in the Unlisted set (block 526). The wireless
device then returns to block 514 and waits for the next measurement
period.
[0053] As an example, the wireless device may sort the cells in the
Unlisted set such that the latest detected cells are at the top and
the oldest detected cells are at the bottom. Cells with the same
detection time are sorted by their received signal quality, with
cells having higher received signal quality being placed on top.
With this structure, the wireless device may sort the cells (if
any) detected in the current measurement period based on the
received signal quality for these cells, e.g., from highest to
lowest. The wireless device then removes cells currently stored in
the Unlisted set that are also detected in the current measurement
period. The wireless device then purges as many additional cells as
needed, starting from the bottom of the Unlisted set. The wireless
device then places the detected cells for the current measurement
period (sorted by received signal quality) at the top of the
Unlisted set.
[0054] The searches performed by the wireless device are dependent
on the pilot and synchronization information transmitted by each
cell in the system. Exemplary list and full searches for W-CDMA are
described below.
[0055] FIG. 6A shows a frame structure used by W-CDMA. Data is
transmitted in frames. Each frame has a duration of 10 ms or 38,400
chips, where each chip has a duration of 260 nano seconds (ns).
Each frame is further partitioned into 15 slots that are given
indices of 0 through 14. Each slot has a duration of 0.67 ms or
2560 chips.
[0056] FIG. 6B shows the transmission of the synchronization
channel (SCH) and the primary CPICH on the downlink by each cell.
The SCH includes a primary SCH and a secondary SCH. The primary SCH
carries a fixed 256-chip primary synchronization code (PSC)
sequence in the first 256 chips of each 2560-chip slot. All cells
in the system use the same PSC. The secondary SCH carries a
different 256-chip secondary synchronization code (SSC) sequence in
the first 256 chips of each slot in one frame. Fifteen different
SSCs are used for the fifteen slots of one frame, and the SSC for
each slot is selected from among a set of 16 SSCs available in the
system. Each cell is assigned one pattern of 15 SSCs and uses this
SSC pattern for the secondary SCH for each frame. The PSC, SSCs,
and SCH are described in a document 3GPP TS 25.211, entitled
"Physical Channels and Mapping of Transport Channels onto Physical
Channels (FDD)," which is publicly available.
[0057] Each cell is assigned one primary scrambling code selected
from among 512 primary scrambling codes available in the system.
The 512 scrambling codes are arranged into 64 groups, and each
group contains eight scrambling codes. Each scrambling code group
is associated with a different SSC pattern. The 64 scrambling code
groups are associated with 64 different SSC patterns. Each cell
transmits a continuous pilot on the primary CPICH using the primary
scrambling code assigned to that cell.
[0058] The wireless device may perform full searches to detect for
the presence of cells and ascertain the timing of these cells. The
full search for W-CDMA may be performed using a three-step search
process.
[0059] In step one, the wireless device searches for the PSC by
correlating the received samples at the device with the 256-chip
PSC sequence at different time offsets (e.g., at every half-chip,
or at 5120 different time offsets in one slot). The PSC is detected
for each time offset in which the correlation result is
sufficiently high (e.g., exceeds a predetermined threshold). The
wireless device uses the PSC to detect for the presence of a cell
and to ascertain the slot timing for the cell. If the cells in the
system are asynchronous, then the PSCs for these cells may be
detected at any time offset.
[0060] In step two, the wireless device determines the pattern of
SSCs used by each cell for which the PSC has been detected. The
wireless device determines which one of the 16 possible SSCs was
received for each of 15 consecutive slots. The 16 possible SSCs and
the PSC are orthogonal to one another. The wireless device can thus
correlate the received samples for a given slot with each of the 16
possible SSCs to determine which SSC was used for that slot. The
wireless device obtains 15 SSCs used for 15 consecutive slots. The
64 possible SSC patterns are selected such that no SSC pattern is a
cyclic shift of any of the other 63 SSC patterns or any non-trivial
cyclic shift of itself. Because of this property, the wireless
device can ascertain which one of the 64 possible SSC patterns was
transmitted based on 15 SSCs detected in any 15 consecutive slots.
The wireless device can determine frame timing and the scrambling
code group used for the cell based on the detected SSC pattern.
[0061] In step three, the wireless device determines the scrambling
code used by each cell for which the SSC pattern has been detected.
Since each SSC pattern is associated with a specific group of eight
scrambling codes, the wireless device evaluates each of the eight
scrambling codes to determine which one was used by the cell. For
this evaluation, the wireless device correlates the received
samples with each scrambling code, accumulates the energies over
some number of chips, and compares the accumulated energy against a
threshold.
[0062] The full search described above can detect for cells whose
identities are known to the wireless device (e.g., sent by the
system) as well as cells whose identities are not known to the
wireless device. Cells detected by the full search and included in
the neighbor list are placed in the Known Timing Neighbor set.
Cells detected by the full search and not included in the neighbor
list are placed in the Unlisted set.
[0063] A signal transmitted by a given cell may reach the wireless
device via one or multiple signal paths. Each signal path is
associated with a propagation delay and a complex channel gain. The
wireless device typically employs a rake receiver that includes
multiple (M) demodulation elements (which are often called
"fingers") and one or more searchers. The searcher(s) search for
strong signal instances (or multipaths) from cells in the system.
The M fingers are assigned to demodulate and process up to M
strongest multipaths for the serving cell(s).
[0064] The wireless device performs list searches to identify the
strong multipaths for cells with known timing. For a list search
for a given cell, the wireless device correlates the received
sample with the known scrambling code for that cell at different
time offsets, which may be selected based on the known timing for
the cell. The wireless device performs the list searches for the
serving cell(s) at every measurement period of P.sub.1 ms to
identify up to M strongest multipaths for the serving cell(s). The
wireless device may then assign a finger to each of these
multipaths. The wireless device performs list searches for
non-serving cells with known timing to determine the received
signal quality for these cells. The received signal quality for
each neighbor cell may be determined by the summing over all
multipaths detected for the cell. The wireless device can perform a
list search more quickly than a full search since the detection for
the PSC and SSCs can be omitted.
[0065] FIG. 7 shows a block diagram of an embodiment of wireless
device 120x, which is one of the wireless devices in FIG. 1. On the
downlink, an antenna 712 receives downlink signals from base
stations (or cells) and provides a received signal to a receiver
unit (RCVR) 716. Receiver unit 716 conditions (e.g., filters,
amplifies, and frequency downconverts) the received signal,
digitizes the conditioned signal, and provides received samples. A
demodulator (Demod) 718 processes the data samples and provides
demodulated data. A decoder 720 then deinterleaves and decodes the
demodulated data and provides decoded data. The processing by
demodulator 718 and decoder 720 is typically different for
different radio access technologies. For cell measurement in
W-CDMA, demodulator 718 may perform the list and full searches for
serving and non-serving cells. For example, demodulator 718 may
process (1) the primary SCH to obtain slot timing of a cell, (2)
the secondary SCH to obtain frame timing and the scrambling code
group for the detected cell, and (3) the CPICH to measure the
received signal quality for the detected cell. For data
demodulation in W-CDMA, demodulator 718 may perform descrambling
with the primary scrambling code assigned to a cell being received,
despreading with orthogonal variable spreading factor (OVSF) codes
for the channel being processed, data demodulation, and so on.
[0066] On the uplink, data (e.g., measurement reports) to be
transmitted by wireless device 120x is processed (e.g., encoded and
interleaved) by an encoder 740 and further processed (e.g.,
modulated) by a modulator (MOD) 742 in accordance with the
applicable radio access technology (e.g., W-CDMA). A transmitter
unit (TMTR) 744 conditions the modulated data to generate an uplink
signal, which is transmitted via antenna 712 to one or more base
stations for the serving cell(s).
[0067] A controller 730 directs operation of various processing
units within wireless device 120x. A memory unit 732 stores data
and program codes used by controller 730 and other processing
units.
[0068] For cell measurement, controller 730 maintains various cell
sets, manages transition of cells among these sets, and directs
searches for cells at the appropriate rates and time. Controller
730 may implement processes 400 and 500 in FIGS. 4 and 5,
respectively, and/or other processes for cell measurement.
Controller 730 may receive various types of information from other
processing units such as, for example, measurement results for
cells (e.g., from demodulator 718), the active set and neighbor
list sent by the system (e.g., from decoder 720), timing
information for the wireless device (e.g., from a timer 734), and
so on. Controller 730 may populate the Active set and the Unknown
Timing Neighbor set with the cells in the active set and the
neighbor list, respectively. Controller 730 may move the cells
among the various sets based on the measurement results obtained
for these cells and may generate measurement reports, as necessary.
Timer 734 may maintain a timer for each cell set and provide an
indication whenever the time for the set expires, indicating that
measurements for the cells in that set are to be made. Controller
730 may direct searches for cells in each set whenever the timer
for that set expires, as indicated by timer 734.
[0069] The cell measurement techniques described herein may be
implemented by various means. For example, these techniques may be
implemented in hardware, software, or a combination thereof. For a
hardware implementation, the processing units used to perform cell
measurements may be implemented within one or more application
specific integrated circuits (ASICs), digital signal processors
(DSPs), digital signal processing devices (DSPDs), programmable
logic devices (PLDs), field programmable gate arrays (FPGAs),
processors, controllers, micro-controllers, microprocessors, other
electronic units designed to perform the functions described
herein, or a combination thereof.
[0070] For a software implementation, the cell measurement
techniques may be implemented with modules (e.g., procedures,
functions, and so on) that perform the functions described herein.
The software codes may be stored in a memory unit (e.g., memory
unit 732 in FIG. 7) and executed by a processor (e.g., controller
730). The. memory unit may be implemented within the processor or
external to the processor, in which case it can be communicatively
coupled to the processor via various means as is known in the
art.
[0071] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *